The classy, cordial Scottish actor, once known for playing relatively decent sorts in “The Chronicles of Narnia,” “Atonement,” “The Last Station,” “X-Men: First Class” and even “Wanted” and “The Last King of Scotland,” has gone alarmingly dark recently.

He got up to some terrible criminal and sexual shenanigans in “Trance” last year. In the new “X-Men: Days of Future Past,” his once nurturing and forthright Charles Xavier has let both himself and his school for young mutants fall apart while he shoots up drugs and gives up his psychic powers.

Those were just prologues, however, to how McAvoy misbehaves in the appropriately titled “Filth.” Adapted from a novel by “Trainspotting’s” Irvine Welsh, the film introduces us to Edinburgh P.D.’s Detective Sergeant Bruce Robertson.

Thanks, Edinburgh, but you can keep him. The guy’s a contempt-spewing, power-abusing, controlled substance-shoveling, manipulative monster. His key goal in life is to sabotage his colleagues for a promotion that he believes will help him win his ice queen ex-wife back — and that’s the noblest thing about him.

“He’s incredibly smart, funny and kind of evil,” notes McAvoy, a 35-year-old married father of a young child. “I was enjoying all that when I started reading the script, but I needed more, needed it to go somewhere else. Then I turned a few pages, and I got given something more.

“I started to realize that all was not well with Bruce,” he says of director Jon S. Baird’s screenplay adaptation. “He’s spiteful and hateful and full of bile, but these are all symptoms of his mental sickness. He’s bipolar and he’s exacerbated that with massive drug and alcohol abuse. He’s paranoid, delusional; he’s not a well man. I’m not saying that when I read that, I forgave him, but it definitely gave me empathy and a deeper interest in what he was about.”

Though a fan of Welsh’s books about self-destructive Scots, McAvoy had not read the “Filth” novel before signing on to the movie, and avoided it while shooting, too.

“The director asked me not to,” he explains. “The book doesn’t have a through-line the way the script does, which is basically Bruce’s mental falling apart.”

McAvoy acknowledges that’s a stab toward audience empathy, something the movie otherwise eschews. Even McAvoy, who’s been grooving on this hard stuff lately, had trouble playing one of the detective’s meanest scenes.

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“The only time I ever had qualms was the first day of the shoot, when Bruce blackmails a 15-year-old girl into giving him oral sex,” he reveals. “I just felt really, really bad about being in that scene. Even though the girl playing that part was in her 20s, it was a really dark feeling to be victimizing somebody like that.

“Other than that, I was actually quite happy to go for it full-on,” he admits.

The feeling was 180 degrees different on McAvoy’s first day shooting “Days of Future Past.” He got to act with Patrick Stewart, the man who played the same character in the first three X-Men movies.

“I’ve got one scene with Patrick in the film,” he happily says. “It was my first day, which happened to be his last day. I was lucky enough to get a kind of face-to-face, nose-to-nose Charles Xavier-off thing going, which I never thought I’d get in a million years when I started on ‘First Class.’ I never thought we’d be in the same movie together, let alone the same scene. It was quite interesting to do my Charles at him and him to do his Charles at my face. It was good.”

McAvoy’s sweet nature, fine manners and Royal Scottish Academy of Music and Drama-trained smarts convince most who meet him that he comes from a genteel background. Not the case. He was raised by his grandparents on the wrong side of the River Clyde.

“I grew up in a very rough part of Glasgow, but I was not a bad kid,” McAvoy says. “I didn’t get into too many run-ins with the police — certainly nobody as f---in’ terrifying as Bruce Robertson. I suppose it’s because I’ve played a lot of posh English people that informs people’s opinions of me. But yeah, I come from an area of Glasgow that’s on a par with the parts of Edinburgh that Irvine Welsh writes about.”

McAvoy’s professional descent into hell has been quite refreshing for him, and he hopes adventurous filmgoers will appreciate the effort.

“I don’t go chasing scripts, I wait for the movies to come to me and that’s just the way it’s worked out,” he explains. “I feel really lucky to have worked with people who are doing films that are bold and risky because they challenge audiences and don’t just make it easy for them. It’s just so rare that movies are even allowed to do that; filmmakers and actors are usually preoccupied with being safe and walking a middle road that doesn’t alienate audiences too much.

“But I like that, though it’s difficult for an audience to go with, hopefully it’s a more revelatory journey for them as well. It’s also what pushes you on as an actor and allows you to learn more about, not just what you’re doing, but storytelling.”